The invention relates to a monitoring and measuring device of the insulation of an electrical network with respect to earth, comprising an input circuit of a reference AC signal to the network to be monitored, and an electronic detection circuit designed to supply a measurement signal significant of the insulation status of the network in reply to the sending of the reference signal, said measurement signal being proportional to the ground fault impedance Z formed by a leakage resistor Rf, and a leakage capacitor C whose capacitance depends on the characteristics of the electrical network, the detection circuit comprising discrimination means of the components of said measurement signal, having a first component being a function of the current IR flowing through the leakage resistor Rf, and a second component being a function of the current IC flowing through the leakage capacitor C.
According to a prior art device, a generator can input a low frequency reference AC voltage to the electrical network via a filter. The leakage current flowing in the ground loop circuit is measured at the terminals of a measuring resistor. The voltage at the terminals of the measuring resistor constitutes in fact the image of the sum of the first partial current via the leakage resistor Rf, and of the second partial current via the leakage capacitor of the network. Direct measurement of the insulation at the terminals of the measuring resistor consequently gives a wrong indication of the insulation status of the electrical network. An auxiliary compensating circuit in some cases enables the influence of the leakage capacitance of the network to be taken into account, but correction of the measurement assumes that the value of this capacitance is previously known. It is difficult to calculate the value of this leakage capacitance accurately as it varies greatly with the configuration of the network to be monitored, notably according to the number of feeders in operation. The voltage measurement at the terminals of the measuring resistor is furthermore often disturbed by interference generated by radiation or conduction over a very wide frequency range. This state-of-the-art insulation monitoring principle can only be applied to a network with isolated neutral.
The document FR 1,586,914 also illustrates measurement of the electrical network insulation by applying a current of a frequency lower than that of the network, and by distinguishing the current flowing respectively through the leakage resistor and capacitance. Electronic detection circuits however remain complicated and are subject to the influence of interference which can give a wrong indication of the insulation status.
The object of the invention is to achieve an insulation monitoring and measuring device protected from interference, and capable of giving a reliable indication of the insulation status of the network regardless of the neutral system chosen, and of the leakage capacitance of the network.